Visual Stress and dyslexia are disorders of different parts of the brain.
Dyslexia is a cognitive processing difficulty; Visual Stress is a disorder of the visual processing system. Obviously this is a complex subject and research is still ongoing, but essentially the two disorders affect different areas of the brain. Visual Stress is common in dyslexia, but does not cause it. About 35% of dyslexics suffer from Visual Stress. It is also fairly common (though less so) in other specific learning difficulties, especially dyspraxia, ADD/HD and autistic spectrum disorders - the autistic author Donna Williams, for example, is well-known for her pink tinted lenses. Visual Stress occurs to varying degrees in about 25% of the population, so it is more common than dyslexia (usually quoted at around 10%); and severely - to the extent that literacy is significantly impaired- in about 5%.
What are the symptoms?
For those who don't know, Visual Stress is a condition that causes headaches and sore eyes, skipping words or lines, hesitant reading with many errors and poor comprehension, and perceptual distortions including movement of letters and words on the page, flickering shadows and coloured "halos", and "rivers" of white running down it. Visual Stress itself is thought to be caused by specific wavelengths of light (they differ with each individual, although certain parts of the spectrum cause more problems than others) that over-stimulate cells in the Visual Cortex, and its effects can be reduced or eliminated by simply placing a precision coloured filter over the page or by wearing precision tinted lenses.
Coloured Overlays don't cure dyslexia
Websites which claim that overlays "cure dyslexia" are not helpful in this respect. As mentioned above Visual Stress is not dyslexia; and secondly coloured overlays don't "cure" Visual Stress any more than spectacles cure short-sightedness: they just deal with the symptoms. Over-simplified claims that are made for marketing purposes and have no basis
Call a spade a spade
We all know what a common cold is, and we know what treatments are available - although the jury may be out on whether they actually work! It's the same with Visual Stress. As well as sometimes being called "Visual Dyslexia", it's also called "Irlen Syndrome ®" (the registered trade mark of the Irlen Corporation), "Meares-Irlen", and "Scotopic Sensitivity", which was the term originally used to describe the condition by Helen Irlen when she started marketing coloured overlays and tinted lenses in the 1980′s. "Scotopic Sensitivity" is in fact a scientifically inaccurate term, as it refers erroneously to the spectral response of the rod system in the retina. "Visual Dyslexia" is a term favoured particularly by some behavioural optometrists. Behavioural Optometry has a valuable contribution to make to Vision Science, but is muddying the patch with this terminology (which is possibly promoted by certain individuals to distance themselves from scientists who query their claims). Visual Stress is actually a neurological condition of the visual cortex. If we don't even agree on what it is called, how are we ever going to understand and treat it?
There are two differing theories as to the causes of Visual Stress. The one mentioned above is called "Cortical Excitablity" and is proposed by Professor Arnold Wilkins of Essex University. There is another theory called "Magnocellular Deficit", which suggests that the problem is caused by a weakness in the transient (Magno) processing cells in the Visual Cortex. This is the theory put forward by Professor John Stein of Oxford University who heads up the Dyslexia Research Trust. Both these academics are eminent scientists doing valuable work and bringing great benefit to the community. The weight of evidence would appear to be in favour of the Essex research, but the fact that scientists disagree on the "Why" may not encourage public acceptance of the "What".
It's not Always Visual Stress
The symptoms mentioned above can be caused by other problems in the visual system, and so coloured overlays are not always the answer - although often they are. Binocular Instability is quite common (though less so), and there can also be accommodative anomalies (problems with focus). Where reading difficulties persist it's always important to refer to an Optometrist. A list of those who specialise in Visual Stress can be found at http://www.crossboweducation.com
Visual Stress is a reality
So whatever we call it today, and whatever its cause, the existence of Visual Stress and the validity of precision tinted filters to treat it is a reality. And it isn't the same as Dyslexia: if coloured overlays effectively treat the visual stress experienced by a dyslexic person, the other processing difficulties associated with that person's dyslexia will remain. It's essential that scientists keep asking questions and testing hypotheses, but while they do let's all call it the same thing and get on with taking it seriously.